The present invention relates generally to the detection and measurement of nonenzymatically glycosylated proteins, and particularly to methods and associated materials for the detection and measurement of proteins that have been nonenzymatically glycosylated in vivo.
Reducing sugars, e.g., glucose, have been shown to react non-enzymatically with protein amino groups to form a diverse series of protein bound moieties with fluorescent and crosslinking properties. These compounds, called advanced glycosylation endproducts ("AGEs"), have been implicated in the structural and functional alteration of proteins during aging and in certain diseases, e.g., long-term diabetes. Several AGEs have been identified on the basis of de novo synthesis and tissue isolation procedures.
The reaction between reducing sugars and the free amino groups of proteins initiates the post-translational modification process called advanced glycosylation. This process begins with a reversible reaction between the reducing sugar and the amino group to form a Schiff base, which proceeds to form a covalently-bonded Amadori rearrangement product. Once formed, the Amadori product undergoes further rearrangement to produce AGEs.
Because these reactions occur slowly, proteins may accumulate significant amounts of Amadori products before accumulating a measurable amount of AGEs in vivo. These AGEs can cause protein crosslinking, which in turn may reduce the structural and/or functional integrity of organs and organ parts, thus ultimately reducing or impairing organ function.
The advanced glycosylation process is particularly noteworthy in that it occurs in proteins with long half-lives, such as collagen and under conditions of relatively high sugar concentration, such as in diabetes mellitus. Numerous studies have suggested that AGEs play an important role in the structural and functional alteration which occurs in proteins during aging and in chronic disease.
Additionally, advanced glycosylation endproducts are noted to form more rapidly in diabetic, galactosemic and other diseased tissue than in normal tissue.
Certain advanced glycosylation endproducts are believed to have in common a characteristic yellow-brown pigmentation, a characteristic fluorescence spectrum and the ability to form protein-protein crosslinks. AGEs form in vivo and have been isolated from naturally glycosylated material. These products are present in low abundance, are structurally heterogeneous and are labile to chemical reduction and hydrolysis. De novo synthesis and isolation procedures have led to the identification of several AGEs, such as 2-(2-furoyl)-4(5)-(2-furanyl)-1H-imidazole ("FFI"); 5-hydroxymethyl-1-alkylpyrrole-2-carbaldehyde ("Pyrraline"); 1-alkyl-2-formyl-3,4-diglycosyl pyrrole ("AFGP"), a non-fluorescent model AGE; carboxymethyllysine; and pentosidine. However, the in vivo formation of AGEs is not limited to these precise chemical compounds, and newly discovered AGEs are addressed herein.
The study of specific AGEs synthesized in vitro in the past has necessitated the use of chemical reduction and hydrolysis procedures. This has left open the possibility that naturally occurring AGEs would include other compounds with alternative structures which differ from the model compounds which have been isolated.
Efforts have also been made to develop antibodies to in vivo AGEs, however no instances of success are known or have been reported. Thus, Nakayama et al., BIOCHEM. BIOPHYS. RES. COMM., 162:2, pp. 740-745 (1989) studied protein bound AGEs and in particular, raised antisera against AGE-KLH derived from in vitro glycosylation. These antisera exhibited high affinity binding, and the serial dilution curves of in vitro-formed AGE-BSA, AGE-HSA and AGE-RNAse A were noted to parallel each other, suggesting that a structure in common among these particular AGE-proteins is recognized by the antisera. Further study to determine whether the structure recognized stems from advanced Maillard reactions or from the early-stage compounds, such as Schiff base adducts and Amadori rearrangement products were conducted using a number of reducing agents. Treatment with a reducing agent did not decrease immunoreactivity, and FFI was not recognized by the antibodies. Importantly, the antibodies prepared and tested by Nakayama et al. were not determined to react with AGEs formed in vivo.
Horiuchi et al., J. BIOL. CHEM., 266(12), pp. 7329-7332 (1991) prepared polyclonal and monoclonal antibodies against in vitro-derived AGE-bovine serum albumin. The Horiuchi et al. antibodies also recognized in vitro-derived AGE-human serum albumin and AGE-hemoglobin, but did not recognize unmodified counterparts. Treatment of these AGE proteins with a reducing agent had no effect on immunoreactivity. Like the antibodies of Nakayama et al., the antibodies prepared by Horiuchi et al. were not determined to react with in vivo-formed AGEs.
Accordingly, despite the facility with which antibodies have been prepared in the art, the reactivity of such antibodies with in vivo-formed AGEs has not been previously achieved. The preparation of such antibodies is desirable as it makes possible the development and implementation of diagnostic and therapeutic protocols addressing the formation of advanced glycosylation endproducts in mammals including humans.
In this context, parent application Ser. No. 07/811,579 discloses the preparation of an antiserum that contains antibodies reactive with in vivo-formed advanced glycosylation endproducts. Among the advanced glycosylation endproducts against which antibodies were raised, the reaction product of hemoglobin and a reducing sugar (Hb-AGE) was included. In addition, data were presented that compared this AGE favorably with HbA.sub.1c in terms of its use as a diagnostic agent.
The present application seeks to present further data cumulative on the activity of Hb-AGE, thereby emphasizing its expanded capabilities. Also, the role of serum- and urinary AGE peptides as markers of disease and dysfunction is further elaborated herein.